Method for producing ethylene copolymers in segmented tubular reactors and utilizing copolymers as a flow improver

ABSTRACT

In a process for preparing ethylene-vinyl acetate copolymers in which a mixture of ethylene and vinyl acetate is polymerized continuously in the presence of free-radical polymerization initiators and, if desired, molecular weight regulators in a cooled tube reactor at a pressure in the range from 1000 to 3500 bar, and the poly-merization initiator is added at the beginning and at two or more points along the tube reactor, and temperature changes in the reaction mixture along the tube reactor, between the first temperature maximum and the last addition of the polymerization initiator, are within a range of not more than 20° C.

The invention relates to ethylene-vinyl acetate copolymers, to a processfor preparing them, to their use as flow improvers, especially inpetroleum middle distillates, such as diesel fuels and light heatingoils, and to fuel compositions comprising them.

Ethylene-vinyl acetate copolymers have been employed for some time asflow improvers in petroleum middle distillates. Their function in suchdistillates is, in particular, to lower the cold filter plugging point(CFPP), which is determined in accordance with EN 116. Preparation ofthe ethylene-vinyl acetate copolymers is generally by polymerization ofthe monomers using the high-pressure technique. US 3,627,838 describes aprocess for preparing pour-point improvers which uses ethylene-vinylacetate copolymers. Ethylene and vinyl acetate are reacted at from 138to 171° C. and under from 48 to 137 bar in a reactor.

DE-A 25 15 805 describes ethylene copolymers, processes for preparingthem, and distillate oils comprising them. The reaction of ethylene andvinyl acetate takes place in an autoclave in cyclohexane as solvent. Thetemperature during the reaction is about 105° C., and the pressure about75 kp/cm².

EP-A-0 007 590 describes petroleum middle distillates of enhancedfilterability. The petroleum distillates contain, for example,ethylene-vinyl acetate copolymers which are prepared solventlessly fromthe monomers at from 100 to 350° C. and under from 500 to 2000 bar. Thepolymers are employed together with a monomer such as vinyl acetate asflow improvers.

The use of stirred autoclaves, in particular, is established in theindustrial preparation of the copolymers. The use of tube reactorswithout backmixing leads in general to more heterogeneous products,since along the tube reactor the reaction mixture passes several timesthrough areas of relatively large increase in temperature following theadditions of initiator. The resulting copolymers generally have a broadmolecular weight distribution.

It is an object of the present invention to provide a process forpreparing ethylene and vinyl acetate copolymers which leads tocopolymers having a narrow molecular weight distribution that can beemployed with advantage as flow improvers in petroleum middledistillates.

We have found that this object is achieved by providing a process forpreparing ethylene-vinyl acetate copolymers in which a mixture ofethylene and vinyl acetate is polymerized continuously in the presenceof free-radical polymerization initiators and, if desired, molecularweight regulators in a cooled tube reactor at a pressure in the rangefrom 1000 to 3500 bar, preferably 1200 bar, and where the polymerizationinitiator is added at the beginning and at two or more points along thetube reactor, and temperature changes in the reaction mixture along thetube reactor, between the first temperature maximum and the lastaddition of the polymerization initiator, are within a range of not morethan 20° C.

It has been found that ethylene-vinyl acetate copolymers havingadvantageous properties, especially narrow molecular weightdistributions, are obtainable if the temperature of the reaction mixturein the tube reactor is held within a very narrow range.

The reaction regime should therefore be as homogeneous as possible alongthe tube reactor. Normally, a polymerization initiator and, if desired,a molecular weight regulator are added to a monomer mixture of ethyleneand vinyl acetate upstream of the tube reactor, and the resultingmixture is polymerized in the tube reactor. At the beginning of thereaction, the temperature first of all increases, before falling againowing to the cooling of the tube reactor. In general, furtherpolymerization initiator is supplied at two or more points along thetube reactor in order to obtain maximum conversions within the tubereactor. Following each addition of the polymerization initiator thereis first of all a further increase in temperature, which then fallsowing to cooling. It has been found that a rapid sequence of additionsof initiator with subsequent short cooling zones along the tube reactorallow a homogeneous reaction regime, i.e. small temperature differences,thereby giving a copolymer having a narrow molecular weightdistribution.

With a given level of cooling, the respective amount of thepolymerization initiator and the sites of the addition of the initiatoralong the tube reactor are preferably chosen so that between the firsttemperature maximum and the last addition of the polymerizationinitiator the average maximum temperature in the reaction mixture is notmore than 15° C., especially not more than 10° C., above the averageminimum temperature.

Prior to the first heating, the reaction mixture has a low temperature,which is present when the monomers are mixed. Following the lastaddition of the polymerization initiator, the reaction mixture is cooledin order to enable the product to be discharged from the reactor.Consequently, in accordance with the invention, the temperature level inthe reaction mixture is left as constant as possible between the firstheating, i.e. the first attainment of the maximum temperature, and thefinal cooling of the reaction mixture.

The average maximum temperature of the reaction mixture is preferably inthe range from 230 to 250° C., with particular preference from 235 to245° C. In particular, the average maximum temperature is about 240° C.The average minimum temperature is preferably about 230° C.

Following the initiation at the beginning of the reactor, the internaltemperature in the tube reactor rises with particular preference toabout 240+/−5° C. The reaction mixture then cools owing to the coolingof the tube reactor (preferably by means of water cooling) to about 230°C. +/−5° C. This is followed by further initiaton to an extent such thatthe temperature of the reaction mixture rises to about 240° C. +/−5° C.Depending on the reactor length, this procedure is repeated a number oftimes in order to obtain a high conversion.

Depending on the desired conversion it is possible to vary the number ofsites along the tube reactor at which polymerization initiator is added.Polymerization initiator is preferably added at 2 to 6, with particularpreference at 3 to 5, sites along the tube reactor.

The pressure in the tube reactor is from 1000 to 3500 bar, preferablyfrom 1200 to 2500 bar.

The length/diameter ratio of the tube reactor is preferably in the rangefrom 10,000 to 50,000, with particular preference from 15,000 to 30,000.

The proportion of vinyl acetate in the reaction mixture is preferablyfrom 15 to 45% by weight, with particular preference from 20 to 35% byweight. The remainder of the reaction mixture constitutes ethylene. Thecopolymer is therefore preferably composed—apart from residues of afree-radical initiator or molecular weight regulator—of ethylene andvinyl acetate.

Free-radical polymerization initiators which can be employed are anydesired such initiators. It is possible to employ peroxides whichdissociate to free radicals, preferably organic peroxides, air oroxygen.

Examples of suitable free-radical initiators are organic peroxides, suchas peresters, perketals and percarbonates. In particular use is made oftert-butyl perpivalates and/or tert-butyl perisononanoate. The lattercan be employed in a molar ratio of from 10:1 to 1:10, preferably from5:1 to 1:5 and, in particular, about 1:1.

The amount of the free-radical polymerization initiator employed ispreferably from 10 to 1000 mol-ppm, with particular preferance from 20to 100 mol-ppm.

Molecular weight regulators which can be employed are any desired andappropriate molecular weight regulators, such as aliphatic and olefinichydrocarbons, aliphatic aldehydes, and so on. Particular preference isgiven to the use of aliphatic aldehydes, especially propionaldehyde. Themolecular weight regulator is preferably added to the reaction mixtureupstream of the tube reactor. It can also be added together with thepolymerization initiator at the various sites along the tube reactor.

The ethylene-vinyl acetate copolymers obtained in accordance with theinvention preferably have a quotient Q of weight-average molecularweight (M_(W)) to number-average molecular weight (M_(n)) of from 2.0 to2.5. Copolymers prepared by known processes generally have a Q of morethan 2.6. The melt viscosity of the copolymers of the invention ispreferably from 40 to 95 cst, with particular preference from 60 to 80cst. The melt viscosity is determined using a rotary viscometer inaccordance with DIN 53019 at 120° C. The Q values are determined bymeans of gel permeation chromatography.

With the known copolymers the melt viscosity is more than 100 cst at120° C. Products known to date and having lower melt viscosities arepoorer in their solubility or else in-their effectiveness.

The reaction time in the process of the invention is preferably from 60to 240 s, with particular preference from 60 to 90 s.

The copolymers of the invention are highly effective flow improvers forpetroleum distillates, especially petroleum middle distillates, andpreferably fuel compositions. They bring about, in particular, a markedreduction in the cold filter plugging point (CFPP) in accordance with EN116. Moreover, the cloud points, or temperatures at which clouding of anadditive-treated fuel begins, are at markedly lower temperatures than inthe case of fuels without additives.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows comparative turbidity measurements for polymers of theinvention and for comparison polymers. In the figure, the clouding inturbidity units TU is plotted against the temperature T (in ° C.).

The invention also relates to a fuel composition comprising a majoramount of a fuel and an effective flow-improving amount of a copolymeras described above. The proportion of the copolymer in this case ispreferably from 0.01 to 5% by weight, with particular preference from0.001 to 0.2% by weight and, in particular, from 0.01 to 0.1% by weight,based on the fuel composition. Particularly appropriate fuelcompositions are middle distillates such as diesel fuels and lightheating oils. The invention is illustrated below with reference toexamples.

EXAMPLES

Preparing the Ethylene-vinyl Acetate Copolymers

The copolymerization of ethylene and vinyl acetate was conducted in acontinuously operated tube reactor having a length of 500 m and aninternal diameter of 40 mm. The reaction pressure was constant at 1500bar. In Example EVA1 the proportion of vinyl acetate was 23% by weight,in EVA2 26% by weight, in EVA3 30% by weight, in EVA4 33% by weight andin the comparative example, Example C1, 30% by weight. The remainder wasaccounted for by ethylene. The free-radical initiator used was a mixtureof tert-butyl perpivalate (TBPP) and tert-butyl perisononanoate (TBPIN)in a molar ratio of 1:1 in solution in isododecane. The amount ofinitiator was 50 mol-ppm. This initiator was added in equal proportionsupstream of the reactor and at three points over the course of the tubereactor. The molecular weight regulator used was propionaldehyde, whichwas added to the original reaction mixture in an amount of 0.8%.

The temperature in the tube reactor was measured by means of a slidablethermometer. Table 1 below indicates for the 5 products tested thestarting temperature upstream of the tube reactor and the maximum andminimum temperatures measured over the course of the tube reactor. Forthe products EVA1 to EVA4 the addition of initiator also took place atthree constant sites in the tube reactor. For the comparative example,Example C1, additional initiation took place only at 2 further points inthe reactor, since the length of the reactor was required to achieve asufficient reduction in temperature in the extended cooling zones. Table1 also indicates the proportion of vinyl acetate (Vac), the viscosity(visc) of the product, in cst, and the value Q.

TABLE 1 No. Vac Visc [cst] product Start Tmax 1 Tmin 1 Tmax 2 Tmin 2Tmax 3 Tmin 3 Tmax 4 [%] at 120° C. Q EVA1 175 240 234 243 227 239 227237 23 70 2.1 EVA2 175 239 231 243 229 242 230 239 26 70 2.1 EVA3 175239 232 241 232 239 231 239 30 70 2.2 EVA4 175 242 229 242 231 240 233243 33 70 2.1 C1 175 245 205 244 206 245 — — 30 300 3.1

In the comparative experiment initiation took place only three times,since the length of the reactor was required for the longer coolingzones.

The product mixture was worked up by pressure release to atmospheric.There was no further working up.

Fuel Compositions

The copolymers of the invention were tested in a range of fuelcompositions based on petroleum middle distillates of commercialEuropean refinery grade. They had the properties indicated in Table 2below:

TABLE 2 MD1 MD2 MD3 MD4 MD5 MD6 Cloud point CP (° C.) −3 −6 −1 −4 −1 +3in acc. with ISO-3015 CFPP (° C.) in acc. 4 −11 4 −5 −5 −2 with EN 116in acc. with ASTM D 833 829 834 830 834 847 4052 density at 15° C.Sulfur content (ppm) 2800 400 300 410 3000 1300 in acc. with EN 24260Distillation in acc. with ISO 3405: initial boiling (° C.) 163 170 159166 166 164 10% boiling point 198 197 203 188 190 209 (° C.) 20% boilingpoint 206 210 224 204 205 229 (° C.) 50% boiling point 259 251 274 262259 279 (° C.) 70% boiling point 298 281 301 302 297 315 (° C.) 90%boiling point 344 322 338 347 345 356 (° C.) 95% boiling point 365 341353 368 367 372 (° C.) End of boiling (° C.) 378 357 364 355 374 383

The 10% boiling point is the temperature at which 10% by volume of theinitial mixture has distilled off, and so on correspondingly.

The ethylene-vinyl acetate copolymers EVA1 to EVA4 obtained above, in50% strength solution in a heavy solvent naphtha solvent, were added tothe above petroleum distillates. For comparison purposes, theethylene-vinyl acetate copolymer obtained in accordance with Example C1(comparative) was employed, which was a customary commercial copolymerin accordance with EP-A-0 007 590 and was likewise 50% strength in heavysolvent naphtha.

The middle distillates were mixed with the copolymers at 40° C. withstirring and then cooled to room temperature. The amount of additiveemployed is shown in Table 3 below. Also shown is the cold filterplugging point (CFPP) determined in accordance with EN 116. Results forfurther middle distillates are set out in Table 4.

TABLE 3 Determining the CFPP [° C.] in accordance with EN 116 in MD1(CP: −3° C., CFPP: −4° C.) Amount added [ppm] MD1 75 100 150 EVA1 −9 −11−14 EVA2 −13 −17 −21 EVA3 −8 −8 −17 EVA4 −7 −8 −10 C1 (comparative) −7−7 −9

TABLE 4 Determining the CFPP [° C.] in accordance with EN 116 MD: MD2MD3 MD4 MD5 MD6 Amount added: 200 ppm 150 ppm 500 ppm 75 ppm 75 ppm EVA1−7 −10 −6 EVA2 −10 −17 −16 −9 EVA3 −27 −11 −18 −8 −6 EVA4 −11 −18 −8 −2C1 (comparative) −23 −6 −15 −5 0

The results of Tables 3 and 4 show that the middle distillates treatedwith the copolymers of the invention as additives show a better CFPPresponse than the middle distillates to which the comparative additivewas added.

Turbidity Measurements

Turbidity measurements were made using a laboratory turbidityphotometer, model LTP 5, from Dr. Lange GmbH, Düsseldorf. The instrumentmeasures the turbidity by the 90° scattered light dual beam method. Thescattered light, deflected diffusely to all sides by the liquid, ismeasured at an angle of 90°. The turbidity measurement by the scatteredlight method is directly and linearly proportional to the particleconcentration and is indicated in turbidity units TU/F.

For the measurements which are to be made the original cuvette wasreplaced by a temperature-controllable measuring cell for which thecontents of the cuvette can be cooled under programmed control at acooling rate of 0.5° C./min with the aid of a PT 100 temperature sensorand external master controller FR 400, program transmitter PD 420 andthermostat UNISTAT from Huber GmbH, Offenburg.

A 1% strength solution in xylene of each of the samples under test wasprepared at 50° C. 15 ml of this solution were introduced at 50° C. intothe temperature-controllable cuvette and cooled to 30° C. The contentsof the cuvette were then cooled at a rate of 0.5° C./min, and theturbidity units were recorded as a function of the temperature. Theethylene-vinyl acetate copolymer EVA 3 and the comparison copolymer C1,both of which have a vinyl acetate content of 30% by weight, were testedcomparatively.

The results are set out in FIG. 1. As can be seen from FIG. 1, EVA3 hasa markedly improved turbidity behavior relative to C1: whereas for C1the cloud point is at 0° C., it was lowered to −5° C. with EVA3, whichresults in an improved solubility behavior of the products of theinvention.

Surprisingly, despite the reduction in the cloud point and the resultingreduction in the number of crystal nuclei which occur, the copolymers ofthe invention were found to have the markedly improved CFPP responsebehavior already described above.

We claim:
 1. A process for preparing ethylene-vinyl acetate copolymersin which a mixture of ethylene and vinyl acetate is polymerizedcontinuously in the presence of free-radical polymerization initiatorsand optional molecular weight regulators in a cooled tube reactor at apressure in the range from 1000 to 3500 bar, and where thepolymerization initiator is added at the beginning and at two or morepoints along the tube reactor in respective amounts, so that the averagemaximum temperature in the reaction mixture is not more than 15° C.above the average minimum temperature between the first heating and lastaddition of the polymerization initiator and temperature changes in thereaction mixture along the tube reactor, between the first temperaturemaximum and the last addition of the polymerization initiator, arewithin a range of not more than 20° C.
 2. A process claimed in claim 1,wherein the average maximum temperature is in the range from 230 to 250°C.
 3. A process claimed in claim 1, wherein the average maximumtemperature is 240° C. and the average minimum temperature is 230° C. 4.A process as claimed in claim 1, wherein polymerization initiator isadded at from 2 to 6 sites along the tube reactor.
 5. A process asclaimed in claim 1, wherein the ratio of length to internal diameter ofthe tube reactor is in the range from 10,000 to 50,000.
 6. Anethylene-vinyl acetate copolymer preparable by a process as claimed inclaim 1 having a Q in the range from 2.0 to 2.5 and a melt viscosity at120° C. in the range from 40 to 95 cst.
 7. A fuel composition comprisinga major amount of a fuel and 0.001 to 5% by weight of a copolymer asclaimed in claim
 6. 8. A process as claimed in claim 1, wherein theamount of vinyl acetate in the mixture is from 15 to 45% by weight.
 9. Aprocess as claimed in claim 8, wherein the amount of vinyl acetate inthe mixture is from 20 to 35% by weight.
 10. An ethylene-vinyl acetatecopolymer preparable by a process as claimed in claim 8 having a Q inthe range from 2.0 to 2.5 and a melt viscosity at 120° C. in the rangefrom 40 to 95 cst.
 11. An ethylene-vinyl acetate copolymer preparable bya process as claimed in claim 8 having a Q in the range from 2.0 to 2.5and a melt viscosity at 120° C. in the range from 40 to 95 cst.
 12. Anethylene-vinyl acetate copolymer as claimed in claim 6, wherein the meltviscosity at 120° C. is in the range of from 60 to 80 cst.
 13. Anethylene-vinyl acetate copolymer as claimed in claim 10, wherein themelt viscosity at 120° C. is in the range of from 60 to 80 cst.
 14. Anethylene-vinyl acetate copolymer as claimed in claim 11, wherein themelt viscosity at 120° C. is in the range of from 60 to 80 cst.
 15. Afuel composition comprising a major amount of a fuel and 0.001 to 5% byweight of a copolymer as claimed in claim
 10. 16. A fuel compositioncomprising a major amount of a fuel and 0.001 to 5% by weight of acopolymer as claimed in claim
 11. 17. A fuel composition comprising amajor amount of a fuel and 0.001 to 5% by weight of a copolymer asclaimed in claim
 12. 18. A fuel composition comprising a major amount ofa fuel and 0.001 to 5% by weight of a copolymer as claimed in claim 13.19. A fuel composition comprising a major amount of a fuel and 0.001 to5% by weight of a copolymer as claimed in claim
 14. 20. A fuelcomposition comprising a major amount of a fuel and 0.01 to 0.2% byweight of a copolymer as claimed in claim
 6. 21. A fuel compositioncomprising a major amount of a fuel and 0.01 to 0.2% by weight of acopolymer as claimed in claim
 10. 22. A fuel composition comprising amajor amount of a fuel and 0.01 to 0.2% by weight of a copolymer asclaimed in claim
 11. 23. A fuel composition comprising a major amount ofa fuel and 0.01 to 0.2% by weight of a copolymer as claimed in claim 12.24. A fuel composition comprising a major amount of a fuel and 0.01 to0.2% by weight of a copolymer as claimed in claim
 13. 25. A fuelcomposition comprising a major amount of a fuel and 0.01 to 0.2% byweight of a copolymer as claimed in claim
 14. 26. A process forpreparing ethylene-vinyl acetate copolymers in which a mixture ofethylene and vinyl acetate is polymerized continuously in the presenceof free-radical polymerization initiators and optional molecular weightregulators in a cooled tube reactor at a pressure in the range from 1000to 3500 bar, and where the polymerization initiator is added at thebeginning and at two or more points along the tube reactor, whereinafter each addition of polymerization initiator except the last additionof polymerization initiator, the temperature along the tube reactorrises and then cools prior to the next addition of polymerizationinitiator, and temperature changes in the reaction mixture along thetube reactor, between the first temperature maximum and the lastaddition of the polymerization initiator, are within a range of not morethan 20° C.